This Article seeks to determine whether the social science literature on the children of lesbians and gay men precludes the government from relying on child welfare considerations to justify same-sex marriage bans and parenting restrictions affecting lesbians and gay men under the highly deferential rational basis test. Under that test, courts must uphold laws and regulations that have any conceivable basis of fact which is rationally related to a legitimate state interest. After comprehensively reviewing the social science literature, the Article concludes that the empirical evidence showing the lack of an association between parental sexual orientation and the psychological and social functioning of children is so conclusive and uniform that there is no conceivable factual basis for suggesting otherwise. The Article, however, also concludes that there are sufficient indicia of possible differences—between the children of lesbian/gay parents and those of heterosexual parents—in matters related to gender attitudes/interests and sexual orientation to satisfy the easy-to-meet factual component of the rational basis test. But this indicia of possible difference does not justify the differential treatment of lesbians and gay men in matters related to marriage and parenting because the government does not have a legitimate interest in either promoting specific gender attitudes and interests or in discouraging same-sex sexual orientations and conduct. At the end of the day, therefore, child welfare considerations constitute impermissible bases for the government’s differential treatment of lesbians and gay men in matters related to marriage and parenting, even under the highly deferential rational basis standard.